1. Field of the Invention
The invention relates to test instruments and, in particular, to methods and devices for magnetographic quality inspection of materials.
This invention can be used in civil engineering and petroleum industry for quality testing of pipes for critical applications, in mechanical engineering for detection of flaws in rolled products, welds of cylinder blocks of heavy-duty diesel engines and for measuring the size of cracks, in ship-building industry for quality testing of welds of ship hulls and reservoirs, including underwater tanks, and in other fields where products are to be tested for continuity defects in not easily accessible locations and when access is possible only from one side.
2. Description of the Prior Art
Known in the art are various methods and devices for magnetographic inspection of materials, wherein the zone to be tested is brought into contact with a magnetic medium and magnetized. The result is that magnetic leakage fluxes penetrate through the surface of the material in places where flaws occur, and are recorded on the magnetic medium. In this manner a magnetogram or "magnetic picture" of the tested material is obtained.
The magnetogram is read out in a line-by-line cyclic manner by a magnetic pickup which produces signals carrying information on the location, size, and type of flaws. In general, electrical signals supplied by the magnetic pickup can be converted into signals indicative of the quality of the material being tested. To this end, electrical signals are displayed on the monitor screen or converted into pulses controlling light indicators and devices marking flaws by paint on the tested material.
But, when the quality of critical products is to be tested, the test has to be substantiated by a document representing the quality of the tested material. This may be roentgenogram or an X-ray photograph of the tested material. It is not easy to obtain a magnetic photograph of the tested material in the course of the non-destructive inspection process by prior art techniques and devices.
Besides, the production engineer dealing with optimization of manufacturing methods, e.g. welding conditions, has to monitor the internal state of the material being treated. For this purpose, the relief of the magnetic field read by the magnetic pickup is to be presented as a color shadow picture indicative of the quality of the tested material.
In this respect, particularly interesting are attempts to develop a device for magnetographic quality inspection of materials, which includes an indication means made as a system converting the magnetogram into a color shadow picture indicative of the quality of the tested material and a printer providing a paper document of the test results. In this case, quantitative assessment of the defects in the tested material can be made on the basis of the indication signals obtained by electronic processing of information read by the magnetic pickup from the magnetogram.
Known in the art is a method of magnetographic quality testing of materials (cf., for example, U.S. Pat. No. 3,341,771, Cl. 324-37), wherein the material to be tested is magnetized, when a magnetic medium is located on the surface of said material. The magnetic medium is then removed, after it has recorded the magnetogram by the action of the magnetizing field, and said magnetic medium is placed in a device for registering the magnetogram. The device amplifies the information contained in the magnetogram and converts this information into electrical signals indicative of the flaws of the material. The device realizing this method features a magnetic pickup adapted for reciprocating motion above the magnetogram, an amplifier, and an indicator. The indicator represents the pulse signal whose shape is indicative of the quality of the material being tested.
But this method and device for magnetographic flaw detection are deficient in that workpieces for critical applications, which have complex shapes, cannot be tested for quality because the magnetogram obtained by this method cannot be directly converted into a ferrogram which is a visual representation of the magnetic relief, that can be printed on paper. Visual examination of the relief of the magnetic recording is necessary in order to detect flaws in complex workpieces, particularly in welds with reinforcement beads. The prior art method and device provide no means for processing the electrical signal, e.g. signal conversion into a color shadow picture, which is important for weld testing since the information content and resolution can be greatly improved.
Also known in the art is a method of magnetographic quality testing of rolled products, e.g. steel billets (cf., advertising booklet "Magnetographie 9.143", published in Federal Germany, 9/81 K, No. 1-81522-1672). The device realizing this method of magnetographic quality testing comprises an electronic indication unit for processing electrical signals read from the magnetogram. Test results can be registered on a paper tape by a print-out device.
This method and device can be used to determine the size and depth of the detected flaw.
However, this device has a limited application field since it can only be used for workpieces with a flat and sufficiently smooth surface. Quality testing of complex objects, e.g. welded joints, cannot be done by this device because it has no means to tune away from noises caused by the irregular surface of the material being tested, e.g. the reinforcement bead of a weld. Besides, this device has no monitor on which the color and shadow picture can be obtained to indicate the quality of the tested material. The color and shadow picture of the internal state of the material is indispensable for quality inspection of workpieces having complex shapes.
Known in the art is a magnetographic flow detector (cf., USSR Inventor's Certificate No. 482,669, Cl. GOIN 27/89, published in "Biulleten Izobreteny" No. 32, 1975), comprising a readout unit composed of a cylinder adapted to rotate about its axis, a magnetic pickup being located on the cylindrical surface thereof, and a mechanism for driving the magnetogram and the cylinder. The indicator of this device is a monitor featuring a scale and time transformation system, which is coupled to the magnetic pickup. This device can be used to provide a color and shadow picture indicative of the quality of the material being inspected. But this device can only supply single frames, which makes it difficult to automate the testing process. In addition, this device has no printout facilities and cannot, therefore, produce a paper document substantiating the results of the quality tests.